Currently there are two operating satellite based positioning systems, the American system GPS (Global Positioning System) and the Russian system GLONASS (Global Orbiting Navigation Satellite System). In the future, there will be moreover a European system called GALILEO. A general term for these systems is GNSS (Global Navigation Satellite System).
For GPS, for example, more than 20 satellites—also referred to as space vehicles (SV)—orbit the earth. Each of the satellites transmits two carrier signals L1 and L2. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) code. Thus, different channels are obtained for the transmission by the different satellites. The C/A code is a pseudo random noise (PRN) code, which is spreading the spectrum over a nominal bandwidth of 20.46 MHz. It is repeated every 1023 bits, the epoch of the code being 1 ms. The bits of the C/A code are also referred to as chips. The carrier frequency of the L1 signal is further modulated with the navigation information at a bit rate of 50 bit/s. The navigation information comprises in particular a timestamp indicating the time of transmission and ephemeris and almanac parameters.
GPS ephemeris and almanac parameters are basically satellite orbit parameters for a short-term polynomial orbit model of the true satellite trajectory. The parameters are maintained and updated at a GPS control server and further updated at the satellites. Based on available ephemeris or almanac parameters, an algorithm can estimate the position of the satellite for any time while the satellite is in the respective described section. The polynomial orbit models have only one degree of freedom, that is, time. The time base for ephemeris and almanac parameters is the GPS time, namely the GPS time-of-week (TOW). The satellite position calculation is basically an extrapolation of the satellite positions along the orbit as a function of time starting from a known initial position. The initial position is also defined by parameters in the ephemeris and almanac data. A time-stamp moreover indicates when the satellite is at the given initial orbital position. The time-stamps are called time-of-ephemeris (TOE) for ephemeris parameters and time-of-applicability (TOA) for the almanac parameters. Both the TOE and TOA are referenced into GPS TOW.
Ephemeris parameters can generally be used only during 2-4 hours for determining the position of a satellite, due to the rather short-term fitting. On the other hand, a better accuracy can be achieved with this short fit than with a longer fit. The achievable accuracy is 2-5 meters. Almanac parameters, in contrast, can be used for a coarse satellite positioning even for weeks, but they are not suitable for the actual accurate positioning due to the poor accuracy resulting from the long-term fit and also from a smaller number of parameters. Ephemeris and almanac data are broadcast from the GPS satellites in a format specified in the open GPS interface control document (ICD) called ICD-GPS-200. Currently, all GPS receivers have to support this format.
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and it detects and tracks the channels used by different satellites based on the different comprised C/A codes. For the acquisition and tracking of a satellite signal, a signal received by a radio frequency (RF) portion of the GPS receiver is first converted into the baseband. In a baseband portion, frequency errors, for instance due to the Doppler effect, are removed by a mixer. Then, the signal is correlated with replica codes that are available for all satellites. The correlation can be performed for example using a matched filter. The correlation values can further be integrated coherently and/or incoherently in order to increase the sensitivity of the acquisition. A correlation value exceeding a threshold value indicates the C/A code and the code phase, which are required for despreading the signal and thus to regain the navigation information.
Then, the receiver determines the time of transmission of the code transmitted by each satellite, usually based on data in the decoded navigation messages and on counts of epochs and chips of the C/A codes. The time of transmission and the measured time of arrival of a signal at the receiver allow determining the time of flight required by the signal to propagate from the satellite to the receiver. By multiplying this time of flight with the speed of light, it is converted to the distance, or range, between the receiver and the respective satellite. Further, the receiver estimates the positions of the satellites at the time of transmission, usually based on the ephemeris parameters in the decoded navigation messages.
The computed distances and the estimated positions of the satellites then permit a calculation of the current position of the receiver, since the receiver is located at an intersection of the ranges from a set of satellites.
Similarly, it is the general idea of GNSS positioning to receive satellite signals at a receiver which is to be positioned, to measure the time it took the signals to propagate from an estimated satellite position to the receiver, to calculate from this propagation time the distance between the receiver and the respective satellite and further the current position of the receiver, making use in addition of the estimated positions of the satellites. The European Satellite Navigation System, Galileo, can be expected to have an ICD of its own. According to the draft “L1 band part of Galileo Signal in Space ICD (SIS ICD)”, 2005, by Galileo Joint Undertaking, the Galileo ICD will be quite close to the GPS ICD, but not exactly the same. There will be Galileo ephemeris and almanac data, and both will be related to a Galileo system time.
A GPS positioning can be performed in three different positioning modes. The first mode is a standalone GPS based positioning. This means that the GPS receiver receives signals from GPS satellites and calculates from these signals its position without any additional information from other sources. The second mode is a network-assisted mobile station based GPS positioning. For this mode, the GPS receiver may be associated to a mobile communication device. The GPS receiver can be integrated into the mobile communication device or be an accessory for the mobile communication device. A mobile communication network provides assistance data, which is received by the mobile communication device and forwarded to the GPS receiver to improve its performance. Such assistance data can be for example at least ephemeris, position and time information. The positioning calculations are performed also in this case in the GPS receiver. The third mode is a network-based mobile station assisted GPS positioning. For this mode, the GPS receiver is associated as well to a mobile communication device. In this mode, a mobile communication network provides at least acquisition assistance and time information via the mobile communication device to the GPS receiver for supporting the measurements. The measurement results are then provided via the mobile communication device to the mobile communication network, which calculates the position. The second and the third approach are also referred to in common as assisted-GPS (A-GPS). If the assistance data comprises a reference position and ephemeris data for a particular satellite, for example, the GPS receiver may determine the approximate satellite position and motion and thus limit the possible propagation time of the satellite signal and the occurring Doppler frequency. With known limits of the propagation time and the Doppler frequency, also the possible code phases that have to be checked can be limited.
Assistance data for A-GPS has been specified and standardized for all cellular communication systems. The delivery of assistance data is build on top of cellular communication system specific protocols, namely RRLP for the Global System for Mobile Communications (GSM), IS-801 for Code Division Multiple Access (CDMA), RRC for Wideband CDMA (WCDMA) and OMA SUPL. The mobile station assisted mode is currently deployed in CDMA networks in the U.S.A. for positioning of emergency calls.
There are many common features in all of the cellular protocols, for example, the supported GPS modes. That is, all cellular protocols support mobile station based GPS, mobile station assisted GPS and standalone GPS. Further, all protocols have a high dependency on GPS. As indicated above, the assistance data that is provided for A-GPS by a cellular communication network may comprise satellite navigation data including GPS ephemeris and almanac data. All cellular protocols for GPS assistance data define to this end ephemeris and almanac data information elements (IE) with only slight differences. The ephemeris and almanac IEs defined in the cellular protocols are practically identical with those defined in the ICD-GPS-200. Thus, they have also the same limitations and expected accuracy as the ephemeris and almanac data which is broadcast by the satellites. This correspondence makes it easy for a GPS receiver to use the assistance data in position calculations, as it requires practically no conversions or extra software. Also a GPS ionosphere model is sent over the cellular link according to all cellular protocols. The GPS assistance data elements are linked to GPS time according to all cellular protocols. Moreover, the acquisition assistance is tailor-made for GPS only and cannot be used for position calculation in the mobile station according to all cellular protocols. Finally, all data elements are indexed in accordance with the GPS satellite constellation according to all cellular protocols.
However, while there are many common features in all of the GPS related cellular protocols, there are also differences. This means that terminal software receiving the assistance data has to either have an adaptation layer for the cellular protocols or support only some of the cellular protocols. Moreover, the differences in the cellular protocols, especially in the message contents, have effects on the A-GPS performance in terms of time-to-first fix and sensitivity.
A further problem is that in order to use the ephemeris or almanac parameters for predicting accurately the expected satellite code phases and Doppler frequencies in the GPS receiver for the initial signal acquisition, the assistance data from the network has to also include an accurate GPS TOW assistance. In GSM and WCDMA networks, an accurate GPS TOW delivery requires deployment of Location Measuring Units (LMU) at every cellular base station, which are able themselves to acquire and evaluate GPS signals. LMUs, however, are expensive and require a continuous maintenance.
Moreover, the current ephemeris and almanac data formats in the cellular protocols are based on formats defined specifically for GPS. Assistance data will also be of importance for Galileo, in order to ensure that the performance of Galileo will be equal to A-GPS. It can be expected that the Galileo ephemeris format will be different from the GPS ephemeris and almanac formats so that the GPS assistance data format can not simply be used for Galileo as well. If Galileo ephemeris is different from GPS ephemeris, the cellular standards have to be augmented with Galileo specific information elements, and the use of Galileo for a positioning requires extra software in the receivers. Moreover, Galileo and GPS may have a different quality of service, that is, the Galileo ephemeris data may be more accurate than the GPS ephemeris data, resulting in a better accuracy of a Galileo-based positioning. Moreover, Galileo and GPS ephemeris parameters may have different life spans. In this case, simultaneous assistance data updating is not possible but assistance data updates need to be scheduled independently for Galileo and GPS.
Thus, there are various problems with the current GPS assistance data.
It has been proposed to augment 3GPP GPS assistance data elements for Galileo signals by modifying the indexing of the ephemeris data elements so that the indexing could also include Galileo satellites. The format of the ephemeris data would then be essentially the same for GPS and Galileo satellites. With this solution, GPS and Galileo assistance data would still be restricted to the limitations of the current GPS ephemeris and almanac data, and also a GPS TOW delivery is still required.
Further, it is known to enhance the accuracy and integrity of orbit models by means of correction data. The European Geostationary Navigation Overlay Service (EGNOS) and the Wide Area Augmentation System (WAAS), for instance, determine GPS correction data, which take account, for example, of GPS signal delays caused by the atmosphere and the ionosphere. The correction data is transmitted via geostationary satellites and the data can be received by suitable GPS receivers and be used for increasing the accuracy of a GPS based positioning. Further, differential GPS (DGPS) corrections had been introduced for mitigating the effect of selective availability. They are suited to remove atmosphere effects and satellite position and clock drifts. WAAS, EGNOS and DGPS corrections are always bound to a single set of ephemeredes, though. When long-term satellite orbital parameters are used instead of normal ephemeris parameters, WAAS, EGNOS and DGPS corrections cannot be used, because they are bound to the normal ephemeris data.